Take the following exchange from "Hammer of God", for instance, when Father Brown confronts the criminal with his deed. Horrified at his discovery, the perpetrator attempts to commit suicide.

[He] threw one leg over the parapet, and Father Brown had him in a minute by the collar.

"Not by that door," he said quite gently; "that door leads to hell."

"How do you know all this?" he cried. "Are you a devil?"

"I am a man," answered Father Brown gravely; "and therefore have all devils in my heart...."

The last line sent shivers down my spine when I first read it. In just one line Chesterton had summarized so succinctly the source of human evil, that is, the human heart. That we are neither angels nor supermen. That we are driven by passion and desires. That even the best among us traverse life dancing a delicate dance between good and evil.